1. Field of the Invention
This invention generally relates to apparatus for internally controlling the bypass flow in a fixed displacement pump in response to external load and flow requirements in a load responsive system.
More particularly, the invention relates to the internal control of pump discharge pressure to a secondary load circuit at a value of 2.5 psi greater than the pump discharge pressure in the standby load responsive condition via a single signal from the load responsive system or systems.
2. Background of the Invention
Load responsive directional control valves with bypass style compensators such as are described in Haussler U.S. Pat. Nos. 3,488,953 and 3,882,896 and in Budzich U.S. Pat. No. 4,159,724 have greatly increased system efficiency by lowering the horsepower requirements with reference to the use of a load responsive bypass style compensator. The arrangement disclosed in my U.S. Pat. No. 5,244,358, the disclosures of which are incorporated herein by reference, which includes low unload, further reduces horsepower loss over the control valves disclosed in the-aforementioned Haussler and Budzich patents by approximately 50%. The horsepower consumption is also reduced by the arrangement disclosed in my U.S. Pat. No. 5,368,061, the disclosures of which are also incorporated herein by reference. The,above-mentioned prior art patents to Haussler and Budzich deal with load sensed directional valves with bypass control to a reservoir. Only my above-identified United States patents deal with pump controlled low unload to a reservoir with variable bypass to the reservoir. It would be beneficial to use bypass fluid for auxiliary functions by replacing the bypass to the reservoir in prior designs with a bypass to secondary load sensitive circuits.
This general type of control in the art is known as a priority type flow device of the load sense type. Remote load sensitive priority devices are disclosed in U.S. Pat. No. 3,455,210 to Allen; U.S. Pat. No. 4,043,419 to Larson et al.; and United Kingdom Patent No. 2,238,355.
The function of the remote load sensitive priority devices described in the Allen and Larson et al. patents and the United Kingdom patent is a valve or a pump containing a hydrostat. A hydrostat is a device well-known in the art to provide equal pressure sensitive areas offset by a load spring of a fixed value and is spring loaded to the open position in reference to the valve inlet and the priority valve outlet port which is connected to load. Fluid flow cannot be diverted to the secondary through the hydrostat until the priority pressure drop exceeds the set spring force on the hydrostat thereby diverting the excess flow to the secondary circuit. The net result, depending on the manufacture, is a minimum 125 psi unloaded condition. The steering control disclosed in the Larson et al. patent may not be used with some known low unload pumps which do not produce sufficient pressure to load the secondary in a plurality of load sensitive valves.